Development of photoreceptor organs in teleosts, Molecular and immunocytochemical studies

Many vertebrate animals are known to have multiple sites for photoreception that are not involved in vision. Extraretinal photoreception is indicated to play a crucial role in early development, physiology and behavior, as well as in later biological processes. This thesis encompasses four studies investigating the neuromorphological, neurochemical and molecular basis for extraretinal photosensitivity in teleosts (bony fishes) during development.

The first study, investigating developing Atlantic halibut, identified the pineal organ as the only photoreceptor organ with a capacity to mediate photic information crucial for embryonic behavioral processes such as vertical migration and hatching.

In the second study, Atlantic herring and cod were investigated, as they have very different early life strategies: during early development the herring is demersal (bottom-dwelling), wheras the cod is pelagic. In both species, extraretinal photosensory organs develop before the retina. The temporal difference was more pronounced in the demersal herring embryos than in the pelagic cod. Furthermore, herring embryos possess additional "deep" photoreceptors in the brain, previously not shown in developing teleosts.

In the third study, two opsin (the protein part of the photopigment) genes were isolated from the halibut pineal, and characterized as coding for a UV-sensitive and a green-sensitive opsin. Both are expressed in the embryonic pineal organ. At first, UV-sensitive opsin predominates, but during subsequent development green-sensitive opsin prevails, possibly reflecting the switch from a pelagic to a demersal life.

Finally, in the fourth study the presence of UV-sensitive opsin in extraretinal and retinal photoreceptors was investigated in various teleosts from different habitats and with different life strategies. UV-sensitive pineal photoreceptors were only found in the halibut, but UV-sensitive cones were identified in the retina of halibut, salmon, zebrafish,turbot and three cichlid species.

In conclusion, a general developmental pattern can be outlined insofar that extraretinal photoreceptors differentiate earlier than retinal photoreceptors. An earlier differentiation of extraretinal photosensory structures in species with demersal eggs, as compared to those with pelagic embryonic life, is suggested to reflect (1) an early adaptation to the photic environment encountered by the eggs and larvae, and (2) the general developmental programme of the CNS.